1. Field of Invention
This invention relates to the field of thermal detectors, more specifically bolometers and methods of manufacturing said detectors.
2. Description of Prior Art
Visible light is, at times, either inefficient or inappropriate for viewing. At these times, imaging of infrared radiation (IR) becomes an important alternative. Infrared radiation detectors have military and civilian applications. Military applications include weapon sights for individual soldiers, crew-served weapon sights, sensors for missile seekers and as driving aids in vehicles. Civilian uses include thermal imaging for energy efficiency audits, security, analytical instrumentation such as IR spectrometers, and night-time driving aids for automobiles. Typically, a single detector is part of an array of many detectors referred to as an infrared focal plane array (IR FPA).
Detectors of infrared radiation fall into two classes--thermal detectors and photon detectors. Photon detectors depend upon the interaction of photons of infrared light with the solid state electronic structure of the materials; in essence, they measure the rate at which photons interact with the detector. Examples of photon detectors include photoconductive, photovoltaic, MIS and Schottky barrier devices. This application pertains to the field of thermal detectors.
A thermal detector possesses an electrically sensed property which changes magnitude when the temperature of the detector changes; that is, when it absorbs heat from impinging infrared radiation. A thermal detector is, in essence, a phonon detector: incident infrared photons are converted to lattice vibrations, phonons, which affect some property of the detector material. There are three typical types of thermal detectors: (1) pyroelectric in which a temperature change results in an induced charge, (2) thermocouple in which a temperature difference across the detector results in a voltage due to the Seebeck Effect, and (3) bolometric in which a temperature change results in a change in the electrical resistance.
An ideal infrared focal plane array will have a large responsivity (ratio of output signal to input signal), low noise characteristics, be highly uniform, and be easily fabricated using standard semiconductor manufacturing techniques. For a bolometer the response is greatest in a detector which has a large temperature coefficient of resistance (TCR), which relates how much the resistance changes per change in material temperature) and which exhibits a large ratio of temperature change to incident radiation intensity. The thermal conductance of a sensor is a measure of how quickly absorbed heat is lost to its surroundings, a rate proportional to the temperature difference between the detector and its surroundings. Minimizing the thermal conductance minimizes the rate of heat loss. Heat capacity relates how much the temperature of a detector element changes when a certain amount of energy is absorbed. Minimizing the heat capacity maximizes the temperature change per unit energy. This scheme has a constraint: the thermal time constant, which is the ratio of heat capacity to thermal conductance, must be kept below one-half the dwell time of the system for maximum signal. (In this way, the detector element reaches equilibrium before being sampled.) For a system scanning at 60 Hz the dwell time is 8.3 ms.
The theory of noise limitations in uncooled arrays is a highly controversial subject See, for example, Arthur S. Jensen, "Limitations to Room Temperature IR Imaging Systems," SPIE, Vol. 2020, pp. 340-350, 1993. Dr. Jensen discusses, among other things, the resistance of the detector elements. His conclusion is that increasing the resistance of the detector decreases the thermal noise generated within the element by the biasing current. A resistance in excess of 50,000 ohms is best, 100,000 ohms is preferred. M. Shulz and L. Caldwell in their article, "Non uniformity Correction and Correctability of Infrared Focal Plane Arrays," Infrared Physics and Technology, Vol. 36, pp. 763-777, Aug. 1995, discuss the effect of spatial response uniformity of an array on the thermal imaging resolution of the array. They point out that "the variations may exceed 10% for compound semiconductor FPA's" (HgCdTe, InSb, GeAlAs, etc.). It is well known that it is extremely difficult to produce uniform films of other materials commonly used for room temperature arrays, e.g., vanadium oxide (VOx) and barium strontium titanate (BST). Property variations of 10% or even higher are typical. Patents which disclose the use of these materials do not even discuss uniformity, even though it is vitally important to the performance of a focal plane array.
Many of the materials previously used for focal plane arrays are not easily manufacturable. At this point in time, there is a great effort within the government and industry to improve the manufacturability of these materials--a task which may be beyond even today's advanced technology. A material easily manufacturable in highly uniform films and arrays using standard photolithographic techniques would be a significant advance in the state of the art.
U.S. Pat. No. 3,693,011 discloses a bolometer detector which is formed of 1) an electrical insulator, 2) an electrically resistive zone formed of an ion implanted layer, and 3) associated electrical connections and means by which radiation can be absorbed onto the detector. The electrically resistive layer is formed by implanting metal ions into a glass, alumina, or sapphire substrate to a sufficient dose to cause the resistivity to decrease to 10.sup.5 ohm/square. The implanted region is supported on top of a base which has a cavity underneath the active element. This method does not address the need to miniaturize and pattern individual elements for the purpose of making infrared detector arrays. In fact, the dimensions of the bolometer elements are given in the patent; the sensitive element is about 1 cm square, the hole has a diameter of about 2 cm, and the film which contains the bolometer element is 0.05 cm thick. While the patent recognizes the need for thermal isolation from the substrate, the thermal mass is much higher than if the film were thinner than one micrometer. It is not likely that the metal ion-implanted alumina film could be easily patterned by lithography, especially to features less than 100 micrometers in size.
A problem which is common to much of the prior art is the inherently low resistivity of the metal alloy residual item, commonly known as permalloy, which is used as the sensing material. A low detector resistance is difficult to measure. U.S. Pat. No. 5,300,915 describes a bolometer designed to overcome the inherently low resistivity of the metal. The patent discloses a serpentine pattern to achieve a resistance of only 2500 ohms. Dr. Jensen, cited above, implies that a resistance on the order of 100,000 ohms would be more desirable--obviously unobtainable with nickel-iron (permalloy). The serpentine pattern has a further disadvantage of a low "fill factor", which is defined as the percent of the total detector area which is composed of the active sensing material. U.S. Pat. No. 5,300,915 discloses a fill factor of 75%, which is improved over the prior art. Thus 25% of the space used by the detector is not used for detector operation.
A number of patents attempt to address the issues of detector responsivity and thermal isolation by several similar approaches. They involve some sort of bridge structure, typically fabricated of silicon nitride or polysilicon, upon which a detector element is formed either through doping of the bridge material or through deposition of a separate material such as permalloy (Ni-Fe). None of the patents, however, thoroughly optimize the heat capacity and thermal conductance or even address the uniformity requirement so necessary for producing a useable focal plane array. Furthermore, as is well known in the industry, none of the arrays meet manufacturability requirements.
U.S. Pat. No. 5,260,225 describes a method for fabricating an infrared sensitive bolometer having a sensing element composed of a polysilicon/doped polysilicon bilayer membrane which is suspended over a cavity. This patent recognizes the need to decrease the thermal conductance to the substrate by placing the conductive, sensing layer on a thermally and electrically nonconductive membrane. This structure, however, has several drawbacks. Fabricating stable, unstressed polysilicon films requires high temperature deposition and annealing processes which are generally considered to be incompatible with complimentary metal oxide semiconductor (CMOS) processing. Furthermore, to produce a film with a large TCR requires an intermediate doping level which results in a very high resistivity and is very difficult to obtain reproducibly. For example, a 1% uniformity of a 2%/K TCR would require a 0.4% dopant uniformity.
U.S. Pat. No. 5,288,649 describes a similar type of bolometer structure in which the sensing element is composed of a four-layer structure consisting of a passivation layer, an infrared absorber, an insulator, and the actual sensing material (a variable resistor made of amorphous silicon doped to approximately 1400 ohm cm). The entire structure is suspended by pillars above the substrate.
U.S. Pat. No. 5,528,976 describes a multilevel structure with reflective metal film on the lower level and a resistive layer of vanadium oxide on the top level. IR absorbance is maximized through the interference of light from two layers. The vanadium oxide is supported on a dielectric film of silicon nitride to form a suspended membrane. However, it has proven to be extraordinarily difficult to produce films of vanadium oxide meeting uniformity and reproducibility requirements.
U.S. Pat. No. 4,371,861 describes the general material properties of the thin film sensing element used in several bolometers. The sensing element is a Ni-Fe permalloy film. This patent describes a method of getting around a major problem with metallic films--its inherent magnetic field dependence. The patent discloses that films with thicknesses in excess of 400 angstroms and line widths less than about 15 micrometers have very small resistance dependence upon a magnetic field.
Clearly, a detector utilizing a material which acts as the bridge structure, the thermal insulating layer, and the detector element optimizes both the detectivity and thermal properties desirable in a focal plane array which is highly uniform, inexpensive, compatible with CMOS and other standard semiconductor processes; and which would be manufacturable would be a significant improvement over the prior art.